Like fractional sex, fictional sex is sex you make up to boost your stats (the notches on your belt in other words). Or, it can also be sex you deny ever having, out of fear of having too high of a bed count, or out of embarrassment that the person you had it with was sketchy. Basically, if you lie about whether or not you got laid, it's fictional sex.

Bob: Dude, I nailed Cassandra last night! I'm up to 22 notches on my belt already. How many you got?
Dan: Twenty-two? Yeah right! How many of those 22 are fictional sex? Probably more than half. I'd be surprised if you even nailed ten chicks.

Andrea: I heard you left the party with Bob last night. Does that mean you're up to nine different guys now?
Casandra: Shut up! Nothing happened. We just kissed a bit and I made him take me home. And I'm not up to eight yet either! I've only done like three guys. Honest.
Andrea: Yeah... sure... three guys this month. Sounds like fictional sex to me.